A conventional light-emitting device includes a substrate, two electrodes (an anode and a cathode), an emissive layer (EML) containing a material that emits light upon electron and hole recombination, one or more layers between the anode and the EML, and one or more layers between the cathode and the EML. The one or more layers between the anode and the EML may be hole transporting layers (HTLs), hole injection layers (HILs), or electron blocking layers (EBLs). The one or more layers between the cathode and the EML may be electron transporting layers (ETLs), electron injection layers (EILs), or hole blocking layers (HBLs). For simplicity, any layer between an electrode and the EML may be referred to more generally as a charge transporting layer (CTL). The CTLs in general operate to transport and inject electrons and holes into the emissive layer, where the electrons and holes recombine to produce light.
Such a light-emitting device in which the material that emits light is organic may be referred to as an organic LED (OLED). Such a light-emitting device in which the material that emits light is semiconductor quantum dots (QDs) may be referred to as a quantum dot LED (QD-LED, QLED or ELQLED). As compared with OLED display devices, QD-LED display devices advantageously may have longer lifetimes, be operable at higher current densities such that the luminance of the device is higher, and the light emitted covers a narrower range of wavelengths which produces more saturated colors. Another advantage of using QD-LEDs may include the devices being more readily solution-processable such that expensive vacuum systems are not required during processing.
In QD-LED device configurations in which all the layers in the light-emitting device are planar, the refractive indices of the layers determine the proportion of the light generated in the EML that can be usefully outcoupled, i.e. emitted from the device into air ultimately to be received by a viewer or external device. Commonly used materials in QD-LEDs have a refractive index in the range 1.6-2.0, which limits the maximum outcoupling efficiency. Increasing the outcoupling efficiency is desirable because it enables more efficient overall devices, decreasing power consumption and extending device lifetime.
FIG. 1 is a drawing depicting a cross-sectional view of a conventional light-emitting device structure 100, such as an OLED or QD-LED. A stack of planar layers is disposed on a substrate 101, with the layers including: two electrodes including a cathode 102 and an anode 103, an emissive layer (EML) 104, one or more charge transporting layers (CTL) 105 between the cathode and the EML, and one or more charge transporting layers 106 between the anode and the EML. During operation, a bias is applied between the anode and the cathode. The cathode 102 injects electrons into the adjacent CTL 105, and likewise the anode 103 injects holes into the adjacent CTL 106. The electrons and holes propagate through the CTLs to the EML, where they radiatively recombine and light is emitted.
The emitted light may be outcoupled from the device into air, trapped within the layer stack, trapped within the substrate, or trapped within the electrodes as surface plasmons. Light which is trapped within the layer stack or within the substrate may eventually be absorbed. Only light that outcouples into air may be received by an external viewer or device, and therefore only this light contributes to the overall efficiency of the device 100. The device as described with reference to FIG. 1 may be referred to as a “standard” structure in that the anode is closest to the substrate relative to the cathode. However, the positions of the anode and cathode may be interchanged, and comparable principles are equally applicable to either structure. A device in which the cathode is closest to the substrate may be referred to as an “inverted” structure.
As light is generated in the EML and propagates through the layer stack, reflection will occur at interfaces between the different layers due to differences in optical properties, particularly refractive index, between the different layers. The EML and CTLs typically have similar refractive indices, and accordingly reflection at these interfaces is minimal. However, in configurations in which reflective or partially reflective electrodes are used, which typically is preferred, the optical properties of the CTLs differ significantly from optical properties of the adjacent electrode layers. Accordingly, a substantial amount of the light will be reflected at the CTL/electrode interfaces.
The planar layers and parallel interfaces of the conventional light-emitting device structure 100 produce a device with favourable electrical characteristics. However, because of the difference in refractive indices between the substrate, the layers and the air, the planar layer structure is limited to outcoupling that is approximately 20%-25% of the light emitted in the EML into air, such that the external quantum efficiency of a planar QD-LED is limited to a maximum of about 25%. In an exemplary embodiment of the device, approximately 25%-30% of the light emitted may be trapped in the substrate, 15%-30% of the light may be trapped in the layer stack, 10%-30% of the light may be lost to surface plasmons, and 5% of the light may be absorbed and lost in the device layers.
In comparison to the external quantum efficiency, the internal quantum efficiency of QD-LED devices including cadmium has reached nearly 100%, and the internal quantum efficiency of QD-LED devices that are free of heavy metals, such as cadmium, has reached over 50%. Thus, modifying the materials in the EML for efficiency improvements of the QD-LED device may be limited for QD-LED devices that are free of heavy metals such that increasing the extraction efficiency of QD-LEDs to improve the efficiency is desirable. As compared with modifying the materials which may improve the quantum efficiency by a factor of two, increasing the extraction efficiency of QD-LEDs may advantageously improve the quantum efficiency by a factor of four.
Prior attempts have been made to improve the extraction efficiencies of OLED devices, which have a similarity in basic structure as compared with QD-LEDs. One prior attempt includes controlling the thicknesses of the layers making up the light-emitting device to form a micro-cavity, as set forth in U.S. Pat. No. 7,973,470 (Cok, issued Jul. 5, 2011). Although more light is coupled forward out of the device to increase extraction efficiency, an undesirable effect on the angular emission from the device may occur. Consequently, when the device is used in a display, a change in luminance corresponding to the viewing angle may occur, and when red, green and blue devices are combined to create a white light, a colour shift may occur that corresponds with the viewing angle.
Another prior attempt includes adding texture to a surface outside of the volume enclosed by the two electrodes. For example, a regular or irregular array of micro-optics such as lenses may be included on the substrate of the light-emitting device. These micro-optics may be fabricated directly on the substrate, as disclosed in U.S. Pat. No. 7,535,646 (Chari et al., issued May 19, 2009), or fabricated on an adhesive sheet and subsequently applied to the substrate, as disclosed in US 2017/0110690 (Lamansky et al., published Apr. 20, 2017). Alternatively, the surface of the substrate may be roughened, as disclosed in U.S. Pat. No. 8,941,296 (Okuyama et al., issued Jan. 27, 2015) or an optical grating may be applied to the surface, as disclosed in WO 2017/132568 (So et. al., published Aug. 3, 2017).
Other approaches to address the above problems have proven deficient, such as adding texture to surfaces within the volume enclosed by the electrodes to decrease the amount of light which undergoes total internal reflection (TIR) and the amount of energy which is coupled into surface plasmon modes. For example, U.S. Pat. No. 9,774,004 (Wu et al., issued Sep. 26, 2017) discloses adding a corrugated polymer layer under an electrode which causes all subsequent layers in the light-emitting device to be corrugated to improve the light extraction efficiency. US 2018/0097202 (Forrest et al., published Apr. 5, 2018) discloses using a polydimethylsiloxane (PDMS) stamp to add texture to an organic CTL before an electrode is applied. U.S. Pat. No. 9,318,705 (Birnstock et al., issued Apr. 19, 2016) discloses adding organic protrusions to a CTL beneath the top electrode by vacuum deposition. WO 2015/096349 (Wu, published Jul. 2, 2015) discloses adding particles into one of the two electrodes of the light-emitting device. However, adding substantially non-planar layers may consequently reduce the electrical performance of the devices.
Still another approach includes directly patterning the electrode to improve extraction. US 2015/0179971 (Yamana et al., published Jun. 25, 2015) discloses using mesh structures, U.S. Pat. No. 9,865,836 (Wang et al., issued Jan. 9, 2018) discloses using periodic structures, and US 2018/0083219 (Copner, published Mar. 22, 2018) discloses using photonic crystal structures. However, patterning the electrode requires additional manufacturing techniques that increase the manufacturing complexity of the LED device.